1. Field of the Invention
The present invention relates to a system and method for enabling a node, such as a mobile user terminal in a wireless communications network, to adaptively detect a signal in the presence of interference. More particularly, the present invention relates to a system and method for enabling a node in an ad-hoc packet-switched communications network to detect the presence of a sequence in the presence of noise when the sequence is transmitted.
2. Description of the Related Art
Wireless communications networks, such as mobile wireless telephone networks, have become increasingly prevalent over the past decade. These wireless communications networks include terrestrial cellular telephone networks, satellite-based telephone networks, and terrestrial or satellite wireless data communications networks, to name a few.
When terminals, such as wireless mobile user terminals, in any of these types of communications networks communicate with one another, the communication signal can contain noise. This noise can be caused by interfering signals from other user terminals, as well as other factors such as thermal noise, environmental noise, and so on.
In most applications, fixed synchronization techniques are used to detect noise in a network. That is, a fixed acceptable noise level is set by the receiver. Any signal having a power level below the fixed noise level is considered to be noise, and any signal having a power level above this level is considered to be a valid signal.
However, as can be appreciated by one skilled in the art, noise levels and signal levels are not constant. Furthermore, the known fixed synchronization techniques do not account for these variances
Various other techniques exist for distinguishing a valid received signal from noise. For example, U.S. Pat. No. 6,229,842 to Schulist et al., the entire contents of which being incorporated herein by reference, discloses a system and method for detecting and selecting peaks in a delay power profile (DPP) signal being received by a direct sequence code division multiple access (DS-CDMA) spread spectrum receiver. The system and method calculate an adaptive threshold which is used to determine valid paths in the DPP signal. Specifically, the adaptive threshold is determined by measuring the signal-to-noise ratio of the DPP signal using an iterative process in which a raw estimate and an improved estimate are made of the noise, and the threshold is set to minimize non-detections and false alarms in the path estimation.
Furthermore, U.S. Pat. No. 5,724,384 to Kim et al., the entire contents of which being incorporated herein by reference, discloses a pseudo-noise code synchronization device which employs an adaptive threshold in a spread spectrum receiver. The receiver is capable of performing a stable pseudo noise code synchronization of a received spread spectrum signal by varying the threshold according to the variance of the received spread spectrum signal. In addition, U.S. Pat. No. 5,642,377 to Chung et al., the entire contents of which being incorporated herein by reference, discloses a PN code acquisition system for Code Division Multiple Access (CDMA) Direct Sequence Spread Spectrum (DSSS) systems that adaptively estimates optimal threshold by exploiting the statistics of the signal and noise, and making an optimal decision based on the threshold.
In addition, U.S. Pat. No. 6,049,576 to Magill, the entire contents of which being incorporated herein by reference, discloses a communication system for communicating between a plurality of stations. At the RF transmitters, a synchronization word signal is periodically inserted in a data stream to assist the stations in achieving timing and frequency accuracy to successfully demodulate a received data stream, as well as to minimize the length and duty factor of the synchronization (sync) word and the time required to acquire synchronization. The synchronization word signal is generated from a Kronecker product code and the RF receivers have detectors detecting Kronecker product code synchronization word and achieving synchronization.
As shown in FIG. 1, the system includes a dual-stage coherent matched filter where the product nature of the code has been utilized to reduce the number of required taps. The first stage, stage A, of the matched filtering is accomplished by a tapped delay line DL-1 whose coefficients are matched to the high rate code. In this example it is assumed that there are two samples per chip. Thus, each high rate chip is represented by two taps (C0, C0, C1, C1 . . . C19, C19), and the delay associated with each tap is one-half of a high rate chip duration. The second stage, stage B, of matched filtering operates on the output from summer S1 of the first stage of matched filtering. For this stage, the tap spacing is one low rate chip in duration and there is only one tap per chip (A0, A1, A2, A3 . . . A23). The sampling rate is not reduced in this second stage and it is necessary to have as many shift register stages as twice the length of the product code. The number of taps is greatly reduced with respect to the number that would exist if one were to directly matched filter to the product code in a single stage.
As indicated in FIG. 1, the Magill system further includes a comparator that compares the output of stage B with a fixed threshold. Because the system uses a fixed threshold, it is generally unsuitable for anticipating variations in the input signal. Therefore, it is not suitable in networks which require that the system be adaptable to variations in the input signal.
For example, in recent years, a type of mobile communications network known as an “ad-hoc” network has been developed for use by the military. In this type of network, each user terminal (hereinafter “mobile node”) is capable of operating as a base station or router for the other mobile nodes, thus eliminating the need for a fixed infrastructure of base stations. Accordingly, data packets being sent from a source mobile node to a destination mobile node are typically routed through a number of intermediate mobile nodes before reaching the destination mobile node. Details of an ad-hoc network are set forth in U.S. Pat. No. 5,943,322 to Mayor, the entire content of which is incorporated herein by reference.
More sophisticated ad-hoc networks are also being developed which, in addition to enabling mobile nodes to communicate with each other as in a conventional ad-hoc network, further enable the mobile nodes to access a fixed network and thus communicate with other types of user terminals, such as those on the public switched telephone network (PSTN) and on other networks such as the Internet. Details of these types of ad-hoc networks are described in U.S. patent application Ser. No. 09/897,790 entitled “Ad Hoc Peer-to-Peer Mobile Radio Access System Interfaced to the PSTN and Cellular Networks”, filed on Jun. 29, 2001, in U.S. patent application Ser. No. 09/815,157 entitled “Time Division Protocol for an Ad-Hoc, Peer-to-Peer Radio Network Having Coordinating Channel Access to Shared Parallel Data Channels with Separate Reservation Channel”, filed on Mar. 22, 2001, and in U.S. patent application Ser. No. 09/815,164 entitled “Prioritized-Routing for an Ad-Hoc, Peer-to-Peer, Mobile Radio Access System”, filed on Mar. 22, 2001, the entire content of each of said patent applications being incorporated herein by reference.
As with the conventional types of communications networks discussed above, communication signals being transmitted and received by user terminals in an ad-hoc communications network are also susceptible to noise that results from interfering signals from other user terminals, thermal noise, environmental noise, and so on. Accordingly, a need exists for a system and method which enables a node in an ad-hoc communications network to accurately detect a signal in the presence of noise in such a manner to minimize false alarms in signal detection.